1. Field of the Invention
This invention relates to a method for the production of solid lubricant agglomerates and, more particularly, to a method for the production of rounded hexagonal boron nitride agglomerates within a desired size range and of a predetermined composition and density.
2. Description of the Related Art
Solid lubricants such as hexagonal boron nitride (hBN) powders are blended or clad with matrix forming metal and blended with other additives to form thermal spray compositions used as abradable seals on gas turbine engines, turbochargers, steam turbines and other rotary equipment. Such an application is, for instance, described in U.S. Pat. No. 5,976,695 by K. Hajmrle et al. Among other thermal spray applications using solid lubricant compositions, is protection of compressor disc and blades against fretting as described in U.S. Pat. No. 5,601,933 by K. Hajmrle et al.
For thermal spray and other applications, larger particles than those resulting from conventional hBN production techniques are necessary. By its nature, the synthesis of hBN produces particulates smaller than 10 microns whereas thermal spray applications require larger particles in the range of 20 to 150 microns.
It is well known in the art to produce solid lubricant particles of hBN by hot pressing fine particles of hBN to form large agglomerates and subsequently crushing, milling and classifying the resulting particles to the desired particle size range. This process is expensive due to the high cost of the several manufacturing steps, especially the high temperature hot pressing step. The cost of the product is further increased because of the production of undersize which must be discarded as waste, and oversize particles which must be further crushed and processed. The undersize fine fraction lowers the process recovery substantially.
The final product produced by the “hot pressing” method is relatively soft which causes problems in further processing. For instance, when such material is further processed by hydrometallurgical metal cladding in an autoclave, the particles disintegrate to a high degree and the particle size of the final product is difficult to control. Stronger particles are required in this application. Stronger particles are also an advantage in powder mixes when two or more different powders are mechanically blended. The soft particles prepared by the hot pressing method have to be handled more gently.
The low recovery of the hot pressing process for producing large hBN particles results in high cost product. The narrower the required particle size cut, the higher the scrap rate and the product cost. For that reason, compromises must be made and wider particle size cuts used then those required for a particular process or product. Exact tailoring of particle size is impractical due to high hBN cost. These compromises lead to inefficiencies in subsequent thermal spraying such as low deposit efficiency and low retention of hBN in the spray coating.
Another disadvantage of prior art hBN particles is the angular and irregular shape of the particles, which may cause problems in powder feeding during thermal spraying.
A further disadvantage of the prior art hBN particles is the inflexibility in composition and density of the particles. For instance, addition of fillers, i.e. other solid particles, to hBN composition is limited by the hot pressing process that is carried out at high temperatures and pressures.